Asymmetric write for ferroelectric storage

ABSTRACT

A method of writing to ferroelectric storage medium includes the steps of applying a first write voltage to a ferroelectric layer for writing a first bit in a first polarization direction and applying a second write voltage to the ferroelectric layer for writing a second bit in a second polarization direction opposing the first polarization direction. The first write voltage having a first magnitude, and the second write voltage having a second magnitude being greater than the first magnitude. The ferroelectric layer having a ferroelectric imprint polarization direction, and the first polarization direction being substantially the same as the ferroelectric imprint polarization direction. The ferroelectric medium contains first bits with a first surface area that is substantially equal to second bits surface area. A probe storage apparatus can use this method and ferroelectric medium.

BACKGROUND

The reversibility of the spontaneous polarization makes ferroelectricmaterials promising candidates for use as storage media in futurenon-volatile memory devices. Binary information is stored in the tworemanent polarization states by applying an appropriate switchingvoltage to a ferroelectric capacitor. After poling the capacitor intothe desired state, the polarization is preserved without the applicationof an external field.

Ferroelectric materials can form the basis for data storage devices,where digital “1” and “0” levels are represented by the electricpolarization of a ferroelectric film pointing “up” or “down”. Storagedevices based on a ferroelectric storage medium include FerroelectricRandom Access Memory (FeRAM) and scanning-probe storage systems(“FE-probe”).

In a FeRAM memory cell the storage element includes a thin ferroelectricfilm sandwiched between fixed, conductive electrodes. To write a bit tosuch a cell, a voltage pulse of either positive or negative polarity isapplied between the electrodes in order to switch the internalpolarization of the ferroelectric film to the “up” or “down” state,respectively. To read back the data from the FeRAM cell, a read voltageof a certain polarity (e.g. positive) is applied, which switches thepolarization of the ferroelectric film in cells storing a “0” (“down”polarization), while having no effect in cells storing a “1”. A senseamplifier measures the charge flow that results when the polarizationswitches, so that a current pulse is observed for cells which stored a“0”, but not for cells which stored a “1”, thus providing a destructivereadback capability.

Probe storage devices have been proposed to provide small size, highcapacity, low cost data storage devices. A probe storage device based onferroelectric thin films uses one or more small, electrically conductingtips as movable top electrodes to store binary information in spatiallylocalized domains. Binary “1's” and “0's” are stored in the media bycausing the polarization of the ferroelectric film to point “up” or“down” in a spatially small region (domain) local to the electrode, byapplying suitable voltages to the electrode. Data can then be read outby a variety of means, including sensing of piezoelectric surfacedisplacement, measurement of local conductivity changes, or by sensingcurrent flow during polarization reversal (destructive readout).

In ferroelectric probe storage, a conducting probe tip scans on thesurface of a ferroelectric media to provide an electric field on themedia for write and/or read. The track width of a probe storage isnominally defined by the dimension of the probe tip perpendicular to thescan direction, while the bit length by the distance of the probe tiptravels along the scan direction with a certain applied electricvoltage; both of them are based on an assumption that the electric fieldfrom the probe tip only affects the ferroelectric media underneath thetip footprint. Actual media area affected by an electrically biasedprobe tip, however, is larger than the tip footprint due to fringingelectric field from the tip, which is called electric field bloomingeffect. Ferroelectric imprint causes the blooming on up and down bits tobe an asymmetric size. In other words, the written bit size with acertain polarization state is different from that with opposite state.As a result, a polarization-dependent variation of track width and bitlength is induced, which is undesirable for a ferroelectric probestorage device. A non-uniform track width would lead to erasure ofneighboring tracks and would cause strong variations in the signalmagnitude for opposite bit states (“1” and “0”), which would complicatethe data analysis and reduce the ultimate storage density. A non-uniformbit length will cause an additional jitter, an increase of the bit errorrate and an overall reduced writability/readability especially for highareal density.

Therefore, therefore the compensation of the asymmetric blooming effecton bit size with opposite polarization directions caused byferroelectric imprint in ferroelectric probe storage. There is also aneed to improve writability and write voltage efficiency in otherthin-ferroelectric film based memory systems.

BRIEF SUMMARY

The present disclosure relates to an asymmetric write for ferroelectricprobe storage. In particular, the present disclosure relates to the useof asymmetric voltages to write bits with opposite polarizationdirections. A lower voltage is used to write bits into the preferred(imprinted) polarization direction that is the same as in the as-grownfilm; while a higher voltage for bits with non-preferred polarizationdirection that is opposite to the as-grown direction. By doing that, theinfluence of ferroelectric imprint on the actual area affected by theelectric field blooming is compensated, uniform track width and bitlength between bits with opposite polarization directions are realized,and the writability/readability at high real density is achieved.

One illustrative method of writing to ferroelectric storage mediumincludes the steps of applying a first write voltage to a ferroelectriclayer for writing a first bit in a first polarization direction andapplying a second write voltage to the ferroelectric layer for writing asecond bit in a second polarization direction substantially opposing thefirst polarization direction. The first write voltage having a firstmagnitude, and the second write voltage having a second magnitude beinggreater than the first magnitude. The ferroelectric layer having aferroelectric imprint polarization direction, and the first polarizationdirection being substantially the same as the ferroelectric imprintpolarization direction.

An illustrative ferroelectric medium includes a ferroelectric layerhaving a ferroelectric imprint polarization direction. The ferroelectriclayer includes a plurality of first bits having a first polarizationdirection and each bit having a first surface area on the ferroelectriclayer. The first polarization direction is substantially the same as theferroelectric imprint polarization direction. The ferroelectric layerincludes a plurality of second bits having a second polarizationdirection substantially opposing the first polarization direction andeach bit having a second surface area on the ferroelectric layer. Thesecond surface area is within 10% of the first surface area.

An illustrative probe storage apparatus includes a ferroelectric layer,an array of probes for reading data from the ferroelectric layer andwriting data to the ferroelectric layer and an actuator for moving theferroelectric layer with respect to the probes. The ferroelectric layerhas a ferroelectric imprint polarization direction and a plurality offirst bits having a first polarization direction that is substantiallythe same as the ferroelectric imprint polarization direction and eachbit having a first surface area on the ferroelectric layer and aplurality of second bits having a second polarization directionsubstantially opposing the first polarization direction and each bithaving a second surface area on the ferroelectric layer. The probes areconfigured to apply a first write voltage having the first polarizationdirection and a first magnitude. The probes are configured to apply asecond write voltage having the second polarization direction and asecond magnitude. The second magnitude is greater than the firstmagnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is an isometric view of an exemplary ferroelectric probe storagedevice;

FIG. 2 is a cross-sectional schematic diagram of a portion of aferroelectric storage medium;

FIG. 3 is a schematic diagram of one illustrative embodiment of a probeelectrode, and its mechanical and electrical support structures;

FIG. 4 is a graph that illustrates the application of symmetric writevoltage pulses to the relative size of each bit in a bit track;

FIG. 5 is a graph that illustrates the application of asymmetric writevoltage pulses to the relative size of each bit in a bit track; and

FIG. 6 is a flow diagram of an illustrative method for writing data toferroelectric medium.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

The present disclosure relates to an asymmetric write for ferroelectricstorage. In particular, the present disclosure relates to the use ofasymmetric voltages to write bits with opposite polarization directions.A lower voltage is used to write bits into the preferred (imprinted)polarization direction that is the same as in the as-grown film; while ahigher voltage for bits with non-preferred polarization direction thatis opposite to the as-grown direction. By doing that, the influence offerroelectric imprint on the actual area affected by the electric fieldblooming is compensated, uniform track width and bit length between bitswith opposite polarization directions are realized, and thewritability/readability at high areal density is achieved. While thepresent disclosure is not so limited, an appreciation of various aspectsof the disclosure will be gained through a discussion of the examplesprovided below.

FIG. 1 is a perspective view of an illustrative ferroelectric storagedevice 10. The ferroelectric storage device 10 includes an array 12 offerroelectric heads 14 positioned adjacent to a storage medium 16. Inmany embodiments, the array 14 and the medium 16 are planar and extendgenerally parallel with each other. The array 14 includes a plurality ofelectrodes (also referred to as tips), which are operably coupled toconnectors 18.

The storage medium 16 is coupled to at least one actuator 20, which isconfigured to move the medium 16 relative to array 12 or the arrayrelative to the medium 16. This movement causes the ferroelectric headsto be moved relative to the individual ferroelectric domains on medium16. Each head can include one or more electrodes. To address thedestructive readback of data, one technique reserves at least one sectoron the storage medium 16, which is available for writing data during aread operation. This available sector is thereby used to reproduce thedata, which is being destructively read back. Other techniques rewritethe data to the same domain or to other locations on the media.

FIG. 2 is a cross-sectional schematic diagram of a portion of aferroelectric storage medium 16. In this embodiment the storage mediumincludes a substrate 22, which can be for example Si, a first layer 24which can be for example SrTiO₃ positioned on the substrate, a layer 26which can be for example SrRuO₃ positioned on the first layer, and aferroelectric layer 28 which can be for example lead zirconium titanate(PZT) (PbZr_(x)Ti_(1-x)0₃) positioned on the second layer. Otherintermediate layers may be used to align the structures between thesubstrate and the PZT film, as desired. In addition, the PZT layer canbe doped with other materials, such as lanthanum. Other materials can beused also.

Due to electric field spreading in the ferroelectric film, a thinferroelectric layer is needed for high bit densities. The domain wallstability may improve with thinner films, thereby providing betterthermal stability. A top layer 29 can be included to minimize wear ofthe cantilever electrodes. This material can be liquid or solidlubricant with a high dielectric constant. In one example, the firstlayer 24 has a thickness of about 100 nm, the second layer 26 has athickness in the range from about 50 nm to about 100 nm, and theferroelectric layer 28 has a thickness in the range of 10 to 30 nm. Thetop layer 29 can have a thickness of 1-3 nm.

FIG. 3 is a schematic diagram of one illustrative embodiment of a probehead assembly 30 including a lever 32, and its mechanical and electricalsupport structures 34, designed for scanning probe storage. The probelever 32 includes a pair of thin films 36 and 38 (bilayer), deposited ona substrate 40 containing other supporting films and/or electroniccircuitry, and whose biaxial stress levels are chosen to ensure that thebilayer wants to bend up from the underlying substrate. This can beachieved by choosing the lower film 36 in the bilayer to have morecompressive biaxial stress than the second layer 38 in the bilayer. Thisstressed bilayer is deposited overlapping a sacrificial layer (not shownin FIG. 3), which is removed selectively by a chemical process, so thatthe bilayer will bend up from the substrate when the sacrificial layeris removed. The bilayer has a suitable metal or conductive metal-oxidelayer 42 (referred to as an electrode or tip) attached to it, so thatthe lever substrate can be brought in proximity to the ferroelectricmedia, and the probe metal brought in electrical contact with the mediato allow data reading and writing. The probe metal is chosen to bemechanically hard (to resist wear), to be chemically compatible with themedia (to avoid media or electrode degradation), and to have highelectrical conductivity in both its bulk and surface. Electroniccircuitry can be integrated into the substrate.

In this example, the substrate includes a first layer 44 that supports afirst conductor adhesion layer 46 and an insulating layer 48, of forexample, alumina. A conductor 50 is positioned on the first conductoradhesion layer 46, and a second conductor adhesion layer 52 ispositioned on the conductor 50. A passivation layer 54 is provided onthe insulating layer. A conductor plug 56 provides an electricalconnection between the conductor 50 and the probe 32 through a via inthe passivation layer and the insulating layer. While one electrode isshown in this example, it should be understood that multiple electrodesand other structures could be included in the lever.

This disclosure provides a method and apparatus for providing uniformtrack width and bit length between bits on a ferroelectric film. The useof asymmetric voltages to write bits with opposite polarizationdirections provide uniform track width and bit length between bits on aferroelectric film. A lower voltage is used to write bits into thepreferred (imprinted) polarization direction that is the same as in theas-grown film; while a higher voltage for bits with non-preferredpolarization direction that is substantially opposite to the as-growndirection. By doing that, the influence of ferroelectric imprint on theactual area affected by the electric field blooming is compensated,uniform track width and bit length between bits with substantiallyopposite polarization directions are realized, and thewritability/readability at high areal density is achieved.

Ferroelectric imprint refers to one polarization state being morefavorable than another in a ferroelectric material, which is reflectedby an asymmetry in the coercive voltages between these two states.Imprint behavior has been found to be highly variable from sample tosample, between different ferroelectric materials, and between bulk andthin-film samples, leading to the suggestion of several underlyingmechanisms, including defect-dipole alignment, bulk screening effects,and interface-dominated screening effects, etc. A sputtered PbZrTiO₃(PZT) media consistently has an “up” polarization direction resulting ina shift of the ferroelectric hsyteresis loop along the electric voltageaxis towards the negative side. The positive switching voltage isfrequently smaller than the negative switching voltage, i.e. the film iseasier to switch into its as-deposited polarization direction.

The actual electric field provided by the probe tip distributes in anarea larger than the footprint of the probe tip due to the fringingfield. The distributed electric field gets smaller when it is furtheraway from the head. So the area a ferroelectric polarization that can beswitched by the probe tip is determined by the dimensions in which thedistributed electric field is above the coercive field. As mentionedabove, the positive coercive voltage, “+Vc”, is lower than the negativeone, “−Vc”, indicating the film is easier to switch into itsas-deposited polarization direction. Therefore, a larger area isexpected to be affected by a positive write voltage than by a negativeone. In other words, a larger positive bit (bit+) and a smaller negativebit (bit−) will be written when the write voltages are symmetric,|Vw+|=|Vw−|. FIG. 4 is a graph that illustrates the application ofsymmetric write voltage pulses |Vw+|=|Vw−| to the relative size (topview) of each bit in a bit track 100. The positive bit (bit+) is largerthan the negative bit (bit−).

As discussed above, different written dimensions between positive bits(bit+) and negative bits (bit−) result in variation in both track widthand bit length. When the areal density is low, individual bits are large(e.g., 800 nm) and the influence on the data writability/readability isneglectable. When the areal density increases (e.g., individual bitsbeing 50 nm or less), the influence increases dramatically and willeventually make the written bits extremely asymmetric and cannot be readout properly.

One such an example is when one considers effects associated withinter-symbol interference (ISI), where the state of a bit is influencedlinearly by the state of the neighboring bits during the write or readprocess, or both. For example, a simple cause of ISI upon readback islimited bandwidth in the readback electronics, which can lead to, forexample the signal not returning to zero for a zero-bit surrounded byone bits. This effect would then be exacerbated if the zero bit wasshorter due to the asymmetry effect discussed here. In general there maybe multiple sources of ISI, including bandwidth limits and differencesin the shape of the writing and reading edges of the probe head, but inall cases the described asymmetry will lead to an inability to resolvebits as linear densities increase. One example compares data for 800 nmand 50 nm bit length, with a symmetric write (±7 V) and an extremelyasymmetric write (0V and −7V) where the up polarization state is notwritten at all but just left from its as-deposited state. In thesymmetric case the down domains get “squeezed out” as the linear densityis increased, since the +7V produces much longer bits due to this beingthe preferred polarization direction of the film. In the completelyasymmetric case the opposite happens, with the up domains being squeezedout by the down domains written with −7V.

In many embodiments, the write voltage is lowered for the preferred orimprinted state, in this particular case the positive write voltage, tocompensate the ferroelectric imprint induced bit size asymmetry. Shownin FIG. 5 is a schematic illustration of the write voltage pulse trainfor the compensation. FIG. 5 is a graph that illustrates the applicationof asymmetric write voltage |Vw+|<|Vw−| to the relative size (top view)of each bit in a bit track 101. The positive bit (bit+) is substantiallythe same size as the negative bit (bit−).

Furthermore, an over-compensation appeared to give an opposite asymmetryas the imprint does. Therefore, keeping the negative voltage constantand varying the positive one, is disclosed to find the desired writecondition. In one example, readback data at −3V with nominal bit lengthof 50 nm is presented for write-voltage combinations of +7V/−7V,+3V/−7V, and +1V/−7V (extracted from a data set that included positivewrite voltages from +7V to 0V). The largest signal amplitude, with theleast baseline lift, is obtained for the case of +3V/−7V. Thisdemonstrates the asymmetric write voltage approach helps compensate theferroelectric imprint induced bit size variation and the associated poorwritebility and readability at high areal density.

In many embodiments, the write voltage magnitude in the polarizationdirection opposing the ferroelectric imprint polarization direction is50% greater, or 100% greater or 150% greater than the write voltagemagnitude in the polarization direction the same as the ferroelectricimprint polarization direction. As illustrated in FIG. 4 and FIG. 5,each bit (Bit+ and Bit−) has a surface area equal to the length timesthe width (the bit tracks 100 and 101 are a top view as disposed on asurface of the ferroelectric layer). As the bit size (largest lateraldimension) is decreased to less than 100 nm or 50 nm, the symmetricwrite produces Bit+ with large surface areas and Bit− with small surfaceareas, theses surface areas are at least 50% greater or less than eachother. On the other hand, as the bit size (largest lateral dimension) isdecreased to less than 100 nm or 50 nm, the asymmetric write producesBit+ with a surface area and Bit− with a similar surface area, thesessurface areas are within 20% or less of each other or within 10% or lessof each other or within 5% or less of each other. Thus the asymmetricwrite produces a uniform bit track 101, illustrated in FIG. 5.

FIG. 6 is a flow diagram of an illustrative method 200 for switching aresistance state of a illustrative ferroelectric probe storage. At block201, the method includes applying a first write voltage to aferroelectric layer for writing a first bit in a first polarizationdirection. The first write voltage has a first magnitude. Theferroelectric layer has a ferroelectric imprint polarization direction,and the first polarization direction is substantially the same as theferroelectric imprint polarization direction. Then at block 202, themethod includes applying a second write voltage to the ferroelectriclayer for writing a second bit in a second polarization directionopposing the first polarization direction. The second write voltage hasa second magnitude that is greater than the first magnitude.

Thus, embodiments of the ASYMMETRIC WRITE FOR FERROELECTRIC STORAGE aredisclosed. The implementations described above and other implementationsare within the scope of the following claims. One skilled in the artwill appreciate that the present disclosure can be practiced withembodiments other than those disclosed. The disclosed embodiments arepresented for purposes of illustration and not limitation, and thepresent invention is limited only by the claims that follow.

1. A method of writing to ferroelectric storage medium comprising stepsof: applying a first write voltage to a ferroelectric layer for writinga first bit in a first polarization direction, the first write voltagehaving a first magnitude, the ferroelectric layer having a ferroelectricimprint polarization direction, and the first polarization directionbeing substantially the same as the ferroelectric imprint polarizationdirection; and applying a second write voltage to the ferroelectriclayer for writing a second bit in a second polarization directionsubstantially opposing the first polarization direction, the secondwrite voltage having a second magnitude being greater than the firstmagnitude.
 2. A method according to claim 1, wherein the secondmagnitude is at least 50% greater than the first magnitude.
 3. A methodaccording to claim 2, wherein the second magnitude is at least 100%greater than the first magnitude.
 4. A method according to claim 1,wherein the first write voltage is a positive voltage and the secondwrite voltage is a negative write voltage.
 5. A method according toclaim 1, wherein the first bit has a first surface area on theferroelectric layer and the second bit has a second surface area, andthe first surface area is within 20% of the second surface area.
 6. Amethod according to claim 5, wherein the first surface area is within10% of the second surface area.
 7. A method according to claim 1,wherein the first bit and the second bit have a largest lateraldimension on the ferroelectric layer of less than 100 nm.
 8. A methodaccording to claim 1, wherein the applying a first write voltage stepand the applying a second write voltage step are preformed by a probestorage tip.
 9. A method according to claim 1, further comprisingreading the first bit and the second bit with a probe storage tip.
 10. Aferroelectric medium comprising: a ferroelectric layer having aferroelectric imprint polarization direction; a plurality of first bitshaving a first polarization direction and each bit having a firstsurface area on the ferroelectric layer, the first polarizationdirection being substantially the same as the ferroelectric imprintpolarization direction; a plurality of second bits having a secondpolarization direction substantially opposing the first polarizationdirection and each bit having a second surface area on the ferroelectriclayer, the second surface area being within 10% of the first surfacearea.
 11. A ferroelectric medium according to claim 10, wherein thefirst bit and the second bit have a largest lateral dimension on theferroelectric layer of less than 100 nm.
 12. A ferroelectric mediumaccording to claim 10, wherein the first surface area and the secondsurface area are substantially equal.
 13. A ferroelectric mediumaccording to claim 10, wherein the ferroelectric layer comprises a leadzirconium titanate material.
 14. A probe storage apparatus comprising: aferroelectric layer having a ferroelectric imprint polarizationdirection and a plurality of first bits having a first polarizationdirection that is substantially the same as the ferroelectric imprintpolarization direction and each bit having a first surface area on theferroelectric layer and a plurality of second bits having a secondpolarization direction substantially opposing the first polarizationdirection and each bit having a second surface area on the ferroelectriclayer; an array of probes for reading data from the ferroelectric layerand writing data to the ferroelectric layer; and an actuator for movingthe ferroelectric layer with respect to the probes; the probes areconfigured to apply a first write voltage having the first polarizationdirection and a first magnitude, and the probes are configured to applya second write voltage having the second polarization direction and asecond magnitude, the second magnitude being greater than the firstmagnitude.
 15. A probe storage apparatus according to claim 14, whereinthe first surface area is within 20% of the second surface area.
 16. Aprobe storage apparatus according to claim 15, wherein the first surfacearea and the second surface area is substantially equal.
 17. Aferroelectric medium according to claim 14, wherein the first bit andthe second bit have a largest lateral dimension on the ferroelectriclayer of less than 100 nm.
 18. A ferroelectric medium according to claim17, wherein the first bit and the second bit have a largest lateraldimension on the ferroelectric layer of less than 50 nm.
 19. Aferroelectric medium according to claim 14, wherein the ferroelectriclayer comprises a lead zirconium titanate material.
 20. A ferroelectricmedium according to claim 14, wherein the second magnitude is at least50% greater than the first magnitude.